Heart function is under the control of the autonomic nervous system (ANS) which includes the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). Sympathetic nerve activity has a positive chronotropic effect on heart rate and contractility, which acts to increase stroke volume and cardiac output. Parasympathetic activity has a negative chronotropic effect on heart rate. The balance between SNS and PNS activity provides controlled regulation of heart function. Numerous factors provide feedback to the sympathetic and parasympathetic nervous systems, which can alter the SNS and PNS activity levels and, as a result, heart function. Under abnormal or disease states, disturbance of the coordination of autonomic nerve activity and balance between the SNS and PNS tone can lead to pathologic conditions such as arrhythmias and hemodynamic decompensation.
Therapies involving stimulation of the ANS for treating cardiac conditions have been proposed. Reference is made, for example, to U.S. Pat. Appl. 2003/0100924 to Foreman, et al., U.S. Pat. No. 5,203,326 to Collins, and U.S. Pat. No. 5,658,318 to Stroetmann, et al. Systems for monitoring physiological variables which vary in response to changes in autonomic tone have also been proposed. Such systems may further include delivering a cardiac therapy such as cardiac pacing in response to interpreted changes in autonomic tone.
In U.S. Pat. No. 6,438,409 to Malik et al., a responsive descriptor for measuring autonomic tone is generally disclosed which involves measuring the cosine of the angle between each ventricular depolarization/repolarization vector pair. In U.S. Pat. No. 6,571,122 to Schroeppel et al., a method and apparatus for evaluating heart rate variability in order to forecast a cardiac event is generally disclosed. In U.S. Pat. No. 6,678,547 to Carlson et al., a cardiac rhythm management system that provides an indication of patient well-being based on the autonomic balance between the sympathetic and parasympathetic components of the ANS using time-domain processing of the frequency components of a heart rate interval signal is generally disclosed. U.S. Pat. Appl. No. 2003/0199937 to Carlson et al. generally discloses a cardiac rhythm management system that acquires atrial heart rate variability information as an indication of the autonomic balance between the sympathetic and parasympathetic components of the ANS. U.S. Pat. Appl. No. 2003/0191403 to Zhou et al. generally discloses an implantable medical device and method for assessing autonomic tone using a recurrence score calculated from changes in R-R interval, heart rate variability, patient activity and myocardial ischemia prior to and after an arrhythmia. The recurrence score may then be used to predict the early recurrence of an arrhythmia.
It is apparent that reliable assessment of autonomic tone would be useful in monitoring cardiac condition, predicting pathologic cardiac activity, and controlling cardiac therapies. However, many factors can influence autonomic tone, such as central venous pressure, central arterial pressure, body position, activity level, systemic vascular resistance, and blood volume. Assessment of ANS activity based on a single variable, which is likely to be influenced by other factors as well, may not be reliable at all times. Typically, clinical assessments of SNS or ANS tone are performed over a brief interval of time during which the patient's status is monitored to assure stable and reproducible stresses, which might independently alter the variable being used for the assessment. Such short-term assessments made during an office visit may not be representative of the patient's overall ANS status. A need remains, therefore, for a system and method for reliably assessing a patient's ANS activity for use in patient monitoring, diagnostics, and therapy management.